Insulating film with improved punching characteristics and lead frame using the same

ABSTRACT

The occurrence of burrs and chips in punching an insulating film is suppressed by adjusting an edge tearing resistance of the insulating film. The edge tearing resistance is preferably 50 to 70 kgf/20 mm for the purpose of suppressing the burrs and the chips.

This is a continuation of application Ser. No. 08/249,734, filed May 26,1994, now abandoned.

FIELD OF THE INVENTION

The invention relates to an insulating film with improvedcharacteristics, and a lead frame using the same, and more particularly,to an insulating film to be punched by suppressing the occurrence ofburrs and chips, and a lead frame using the same.

BACKGROUND OF THE INVENTION

So-called pre-fabrication lead frames have been proposed to realize highdensity mounting of semiconductor chips in the form of COL (Chip onLead), LOC (Lead on Chip), etc. This kind of a lead frame is providedwith an insulating film punched to a predetermined configuration andadhesively fixed on an inner lead portion of the frame, wherein theinsulation film is composed of a polyimide-based film having a highheat-proof property and thermoplastic or thermosetting adhesive layerson both surfaces (or one surface) of the polyimide-based film.

In providing the insulating film, an apparatus comprising punches and adie is used to punch an insulating film placed on the die by loweringthe punches, wherein a clearance between each of the punches and the dieis adjusted to suppress the occurrence of chips at the time of punchingthe insulating film and burrs at edges of a punched insulating film.

In the conventional insulating film to be used for the COL lead frame,the LOG lead frame, etc., however, there is a disadvantage in that theoccurrence of chips and burrs can not be suppressed sufficiently, evenif a clearance between a punch and a die is adjusted, because mechanicalcharacteristics of an insulating film to be punched are varied,especially, in regard to an edge tearing resistance. Consequently, thedimensional precision of the punched insulating film is lowered byconcealing an area for bonding wires on the lead frame by burrs andchips. This means that it is impossible for bonding wires to be bondedbetween the lead frame and a semiconductor chip mounted thereon.

There are further disadvantages in the conventional insulating film inthat burrs and chips tend to be accumulated between the punch and thedie, so that the positional precision of fixing a punched insulatingfilm on a lead frame is lowered, and marks of burrs and chips areimpressed on a punched insulating film to be thereby classified into afaulty. This means that cleaning of the punch and die must be doneoften.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide an insulatingfilm with improved punching characteristics and a lead frame using thesame in which the occurrence of burrs and chips are effectivelysuppressed.

It is a further object of the invention, to provide an insulating filmwith improved punching characteristics and a lead frame using the samein which a high dimensional precision is obtained, and no accumulationof burrs and chips is found on a punch and a die in a punchingapparatus.

According to a feature of the invention, an insulating film withimproved punching characteristics, comprises:

an insulating base film having an edge tearing resistance of 50 to 70kgf/20 mm; and

insulating thermoplastic or thermosetting adhesive layers provided on atleast one surface of the insulating base film.

According to another feature of the invention, a lead frame using aninsulating film with improved punching characteristics, comprises:

inner lead portions to be connected to a semiconductor chip;

outer lead portions to be connected to external circuits; and

insulating films punched to being a predetermined configuration, thepunched films adhesively fixed to the inner lead portions;

wherein the punched films comprise insulating base films having an edgetearing resistance of 50 to 70 kgf/20 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail in conjunction withappended drawings, wherein:

FIG. 1 is an explanatory view showing a conventional apparatus forfixing an insulating film to a lead frame,

FIG. 2A is a perspective view showing a semiconductor chip package,

FIG. 2B is a cross-sectional view cut along the line 2B--2B in FIG. 2A,

FIGS. 3A and 3B are explanatory diagrams explaining mechanism for theoccurrence of punching burrs and punching chip in a conventionalapparatus for fixing an insulating film to a lead frame,

FIG. 3C is an explanatory diagram explaining the punching burrs asexplained in FIGS. 3A and 3B,

FIG. 4 is a plan view showing a lead frame using an insulating film inan experiment according to the invention,

FIG. 5 is a graph showing the test results in the experiment,

FIG. 6 is a plan view showing a lead frame using an insulating film in apreferred embodiment according to the invention,

FIGS. 7A and 7B are cross-sectional view showing a punch and a die forpunching an insulating film in the preferred embodiment; and

FIG. 8 is a flow chart for fabricating an insulation film for use inaccordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before explaining an insulating film with improved punchingcharacteristics and a lead frame using the same in the preferredembodiments according to the invention, a conventional apparatus forfixing an insulating film to a lead frame will be explained.

FIG. 1 shows the conventional apparatus for fixing an insulating film toa lead frame. The apparatus comprises a die 2 for placing an insulatingfilm 4 thereon, punches 5 for punching the insulating film 4 andretaining the punched insulating films 4 on the tip plane by vacuum, apunch guide 3 for guiding the punches 5 through guide apertures thereof,a heater 6 for heating a lead frame 1 placed thereon to be apredetermined temperature, a punch holder 51 for holding the punches 5,a shank 52 for vertically moving the punches 5 up and down, and acylinder 53 for generating a moving force to be applied to the shank 52.In some use, a spacer 54 may be provided.

In operation, the insulating film 4 is transferred on the die 2 by antransfer apparatus (not shown), when the punches 5 take an upperposition. At the same time, the lead frame 1 is also transferred to apredetermined position on the heater 6. Then, the punches 5 are moveddown to punch the insulating film 4, so that the punched insulatingfilms 4 are retained on the tip plane of the punches 5 by vacuum, andare lowered to be pressed on the heated lead frame 1 on the heater 6.Thus, a lead frame 1, which is fixed with the punched insulating films4, is obtained. Then, the insulating film-fixed lead frame 1 isprocessed to be adhesively attached to a semiconductor chip forfabricating a semiconductor chip package.

FIGS. 2A and 2B show the semiconductor chip package thus fabricated. Thesemiconductor chip package comprises a semiconductor chip 7 havingelectrodes thereon, and a lead frame 1 fixed with punched insulatingfilms 4, wherein the lead frame 1 is adhesively attached to thesemiconductor chip 7 by the fixed insulating films 4, and a inner leadportion of the lead frame 1 is bonded to the electrodes by bonding wires8, such that a whole structure of the semiconductor chip 7 is sealed toexpose only an outer lead portion of the lead frame 1 by molding resin 9(not shown in FIG. 2A, but shown in FIG. 2B). In the semiconductor chippackage, the punched insulating films 4 are, for instance, polyimidefilms, are layered on the both surfaces with thermoplastic orthermosetting adhesives to provide heat-proof characteristics of 250° to400° C. As understood from the illustration in FIGS. 2A and 2B, theprecision in positioning of the lead frame 1 on the semiconductor chip 7must be high, because the inner leads must be precisely and stablybonded to the electrodes by the bonding wires 8.

FIG. 3A shows an operation in which an insulating film 4, composed of apolyimide film 4a and adhesive layers 4b, is pressed to cause acompressive breakage point 10 in the polyimide film 4a by a punch 5 anda die 2, while substantially no deformation is found in the adhesivelayers 4b, so that punching chips and punching burrs are generated inaccordance with the compressive breakage point 10.

FIG. 3B shows operation in which the adhesive layers 4b of theinsulating film 4 is first punched, and the polyimide film 4a of theinsulating film 4 is then punched, so that no compressive breakage pointis found in the polyimide film 4a. Consequently, burr and chip are notgenerated substantially.

The inventors have confirmed in experiments whether an insulating film 4is punched in operation as shown in FIG. 3A or FIG. 3B is determined bymechanical characteristics of the insulating film 4, among which an edgetearing resistance of the insulating film 4 is a very important factorto suppress the occurrence of punching burrs and punching chips.

FIG. 3C shows four kinds of punching burrs, wherein the burr interruptsthe bonding wire 8 to be bonded to an inner lead of the lead frame 1.

Polyimide films now available for this purpose are listed to indicatemechanical characteristics in the Table 1.

                                      TABLE 1                                     __________________________________________________________________________                  UPILEX-S                                                                            UPILEX-R                                                                            UPILEX-M                                                                             KAPTON-V                                                                             KAPTON-H                              __________________________________________________________________________    TENSILE STRENGTH                                                                            40     25   40     30     25                                    (kgf/mm.sup.2)                                                                ELONGATION (%)                                                                              30    130   60     86     80                                    TENSILE ELASTIC                                                                             900   380   550    352    302                                   MODULUS (kgf/mm.sup.2)                                                        TEARING STRENGTH                                                                            330   750   500˜580                                                                        --     --                                    (kgf/mm.sup.2)                                                                EDGE TEARING  23     40   --     --     --                                    RESISTANCE                                                                    (kgf/20 mm)                                                                   THERMAL EXPANSION                                                                           10     32   10˜25                                                                          --     --                                    COEFFICIENT (PPM)                                                             __________________________________________________________________________

As understood from the table 1, the mechanical characteristics of thepolyimide films range widely. Therefore, punching burrs and punchingchips tend to occur dependent on the deviation from a desired range ofthe mechanical characteristics.

Next, an insulating film 4 with improved punching characteristics and alead frame using the same in the preferred embodiment will be explained.

FIG. 4 shows the lead frame 1 having inner lead portions 1A and outerlead portions 1B, and the insulating films 4 adhesively attached to theinner lead portions 1A of the lead frame 1.

In experiments conducted by the inventors, 30 pieces of insulating films4 are prepared and adhesively attached on the lead frame 1 as shown inFIG. 4. Each piece has two punched insulating films 4, and is dividedinto plural groups, each of which is different in edge tearingresistance from other groups.

Such insulating films are fabricated by the steps as set out below.

First, solvent is added to a starting material such as polyamide acid,etc. to provide varnish. Next, the varnish is coated on a metal mold, aroller, etc., and then heated to be dry. Then, the dried film is peeledfrom the surface of the metal mold, the roller, etc. to providepolyimide-based films. Then, polyimide-based films having adhesivelayers are obtained by coating thermosetting or thermoplastic adhesiveon both surfaces or one surface of the polyimide-based films.

In insulating films with adhesive layers thus fabricated, the edgetearing resistance is changed by adjusting a water component.

For this purpose, a solvent having a water absorbing property is used.Thus, the edge tearing resistance is made to a predetermined value bythe non-volatile amount of the solvent, which is controlled at theheating and dry step.

The measurement of the edge tearing resistance is specified in the JISC2318 6.3.4.

In this standard, five test pieces each having a width of 20 mm and alength of 200 mm are sampled without any concentration on a specificarea in longitudinal and transversal directions whole length and widthof a film to be measured. Each test piece is passed through an apertureof a stationary testing device, and then folded to face each at the bothends. Then, the folded ends of the test piece are chucked to be pulledin a direction at the speed of 200 mm per one minute by a tensilemember. Then, a mean value and a minimum value of pulling forces bywhich the test piece is teared are measured as the edge tearingresistance.

Referring back to FIG. 4, the 30 pieces of the insulating films 4 arepunched to be adhesively fixed to the inner lead portions 1A of the leadframe 1, wherein each piece consist of two insulating films 4,classified into the several groups of different edge tearingresistances, as explained before.

In obtaining the punched insulating films 4, the occurrence percentage %of punching chips is measured as indicated in FIG. 5, wherein a piece oftwo insulating films 4 is determined to have generated punching chips,when the punching chips are found in at least one of the two insulatingfilms 4 for the piece.

As understood in FIG. 5, the occurrence percentage % is decreased, asthe edge tearing resistance of the insulating films 4 is increased. Tobe more concrete, when a polyimide-based film having an edge tearingresistance equal to or greater than 50 kgf/20 mm is used, the occurrencepercentage % is approximately 0%. In this respect, the maximum edgetearing resistance is limited to be approximately 70 kgf/20 mm, becauseit is difficult to fabricate an insulating film composed of a greatamount of residual solvent by heating varnish films coated on molds,rollers, etc. As explained before, the amount of the residual(non-volatile) solvent is reduced, the water or moisture absorptionproperty of an insulating film is lowered to provide a small edgetearing resistance. In accordance with the inventor's experiment, it isconfirmed that an edge tearing resistance of the insulating films 4 ispreferable to be in a range of 50 to 70 kgf/20 mm.

FIG. 6 shows a lead frame 1 having inner lead portions 1A and outer leadportions 1B of a LOC structure, and insulating films 4 ofpolyimide-based films adhesively fixed to the inner lead portions 1A,wherein a semiconductor chip (not shown) is adhesively mounted on theinner lead portions 1A of the lead frame 1 by the insulating films 4. Inthis preferred embodiment, the insulating films 4 have an edge tearingresistance of 50 to 70 kgf/20 mm.

FIGS. 7A and 7B show an apparatus for fixing insulating films to a leadframe which comprises a die 2 on which an insulating film 4 istransferred to be punched, punches 5 for punching the insulating film 4to provide two punched insulating films 4, a heater 6 for heating a leadframe 1 transferred thereon, and a stripper 20 for stripping theinsulating film 4 from the upward moving punches 5.

In operation, the lead frame 1 is transferred to be heated on the heater6. At the same time, the insulating film 4 composed of a polyimide filmhaving an edge tearing resistance of 50 to 70 kgf/20 mm andthermoplastic or thermosetting adhesive layers on both surfaces of thepolyimide film is transferred on the die 2. Then, the punches 5 aremoved down to punch the insulating film 4 (FIG. 7A), and the two punchedinsulating films 4 are retained on tip planes of the punches 5 byvacuum, and lowered to be adhesively fixed to the lead frame 1 (FIG.7B).

It should be noticed that an edge tearing resistance indicates abreakage strength against a compression stress in the thicknessdirection, while a tearing strength indicates a breakage strengthagainst a pulling stress in the direction orthogonal to the thicknessdirection. For this reason, the edge tearing resistance and the tearingstrength are absolutely different, especially, in regard to appliedcompression and tension forces.

An experiment was conducted prior to filing the present application tocheck for a relationship between (a) the physical characteristics oftensile strength and tearing resistance of available insulating filmsand (b) an occurrence rate of burrs at the time of punching theinsulating films. However, the inventors did not find any specificcorrelation of tensile strength and tearing resistance to occurrencerate of burrs. Then, the inventors investigated the method for measuringan edge tearing resistance of an insulating film in which the breakageof the insulating film is similar to that in punching an insulatingfilm.

Available insulating films were measured for edge tearing resistance,and a correlation as shown in FIG. 5 was confirmed. Occurrence of burrsis negligible at edge tearing resistance of 50 kgf/20 mm. and higher.

FIG. 8 shows a process for fabricating an insulating film having an edgetearing resistance of 50 to 70 kgf/20 mm.

Varnish is prepared by adding hygroscopic organic solvent to polyamideacid. A rotating roller having a predetermined diameter is coated withthe varnish. A varnish film (polyamide acid) on the roller is heated anddried.

Then, the varnish film is peeled from the roller to provide a polyamideacid film. With further heating, the polyamide acid film is inverted toa polyimide film. In this stage, the temperature and heating time arecontrolled to adjust the content of the hygroscopic organic solvent (aresidual solvent amount) contained in the polyimide film. Thus, an edgetearing resistance of the polyimide film is made to be 50 to 70 kgf/2mm.

The inventors found that a polyimide film having a large edge tearingresistance also has high flexibility. It is considered that, when agreat amount of water is contained in a polymer precursor, water isincluded among molecules constituting the polymer. When the water isevaporated, fine voids are generated among the molecules and provide thehigh flexibility.

Although the invention has been described with respect to specificembodiment for complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodification and alternative constructions that may occur to one skilledin the art which fairly fall within the basic teaching herein set forth.

What is claimed is:
 1. An insulating film with improved punchingcharacteristics, comprising:a polyimide insulating base film having anedge tearing resistance in a range of 50 to 70 kgf/20 mm; and one ofinsulating thermoplastic and insulating thermosetting adhesive layersprovided on at least one surface of said insulating base film.
 2. A leadframe using an insulating film with improved punching characteristics,comprising:inner lead portions to be connected to a semiconductor chip;outer lead portions to be connected to external circuits; and polyimideinsulating films punched to a predetermined configuration, said punchedfilms being adhesively fixed to said inner lead portions; wherein saidpunched films have an edge tearing resistance in a range of 50 to 70kgf/20 mm.
 3. A lead frame using an insulating film with improvedpunching characteristics, according to claim 2, wherein:each of saidinsulating base films is coated on at least one surface with one of athermoplastic and thermosetting adhesive layer.